Wednesday, April 23, 2014

The Late Triassic in the southern Uinta Mountains

One of my former graduate students (Paul Jensen) and one of my current graduate students (Skyler May) have worked on these Late Triassic Rocks along the south flank of the Uinta Mountains. They are interesting rocks with interesting sedimentary structures. We traveled out there this week to look at them again.

Panoramic view of one of the ships in the Red Fleet along the south flank of the Uintas. The particular rocks we are studying are the rocks that make up the "deck" of the ship and the red cliff above the lower slope. The lower slope is the Chinle Formation, the red cliff and deck of the ship are the Bell Springs Formation, and the upper lighter red rocks forming the top part of the ship are the Nugget Sandstone.

Rippled sandstone with small clay lumps on the ripples. As the water flowed past the clay lumps, they allowed sand grains to accumulate in their flow "shadow" showing that the water was flowing from the upper left to the lower right across this bed.

This layer of sand was covered by a thin layer of mud that dried out and cracked. The cracks were then filled in with more sand. Fossilized mudcracks are common in the Bell Springs Formation.

The massive red cliff sandstone is almost entirely composed of ripple laminated, very fine sand as seen in this photo. The individual ripple layers are 1-3 cm in thickness with ripple wavelengths typically 2-6 cm. In places, a few mud lumps are found in the sandstone, as seen near the top of this photo and some ripples have thin mud drapes.

Occasionally, small fluid escape tepees, as seen here, can be found in the rippled sandstone. In addition, the sand is pock marked with small burrows and bioturbation, seen here and below.

Color and b&w shot of bioturbated portion of the red sandstone cliff. Here the burrowing has almost completely destroyed the ripples, although some can still be seen in a place or two.

The small white rectangular blades and rosettes seen on this sandstone surface are salt casts: places where salt crystals grew in the sand and then later dissolved away. This layer of sandstone is at the top of the red cliff and forms the first layer of the deck of the Red Fleet ship.

Standing here on the surface with the salt casts is Doug Sprinkel of the Utah Geological Survey. Between Doug's feet and hat are a series of sand layers that lie like the shingles on a roof dipping to the left. These were likely deposited on the point bar of a small river.

Above the point bar deposits are layers with small cross-bed sets. The lower set seen here has been truncated by a second set. The contact between the two sets (a few inches above the hammer handle) is wavy and the first sand deposited into the waves has layering that follows the wavy boundary before the second sand set becomes established.

Here is a close up of the contact described above. The lower cross-bed set truncated along the wavy surface. Then the sand filling in the waves, then a thin crinkly bed with a slightly darker color, and finally the second cross-bed set.

Crinkly beds in these sandstones as seen here in this 2004 photo, may be due to algal growth on the sand shortly after it was deposited.

Cross-bedded sandstones about 0.5 m in thickness are separated by crinkly sandstone layers near the top of the red cliff in the Bell Springs Formation. This photo was taken in 2004.

Here the Bell Springs Formation is exposed in Dinosaur National Monument to the east of the Red Fleet section and the big cliff forming sandstone that was characteristic of the formation at Red Fleet is absent. Photo taken in 2012.

Ron Blakey's paleogeographic map of the Bell Springs Formation time period. According to his map, northeastern Utah would have been part of a broad fluvial plain stretching out to the west from the highland areas of Colorado to the ocean and island arc along the western margin of North America.

Friday, April 11, 2014

Remembering 1983: The year of the floods in Utah

Starting in the Fall of 1982, Utah–particularly along the Wasatch front–had record or near record rainfall and precipitation every month from September of that year through June 1983. The mountains in early June looked more like the mountains do in March of a typical year with deep snow still halfway down the slopes. These photos were taken between March and the first week of June in 1983 in Spanish Fork Canyon, Provo, and Salt Lake City. So, as we struggle through the dry years, remember that the wet ones can be just as bad, or worse, in their own way.

In late February or early March 1983, the first sign of the trouble to come was the reactivation of a small landslide in Provo labeled on the geologic map as the Easter Cross landslide. Developers, with the blessing of the city, had allowed lots to be drawn up for sale on the old landslide. I am standing near the active toe of the landslide as it crept ever closer to a home that had just been build on one of the lots.

Up in Spanish Fork Canyon, to the south of Provo, an even bigger landslide started moving. Initially, the railroad folks tried to dig up their rails, smooth things out, and relay the rails over the slowly moving landslide. But when it kept moving, they eventually gave up. Here the slide has grown to a huge size, dwarfing the large earth moving equipment that was called in to stabilize the slide. The Spanish Fork River was now dammed and backing up behind the slide creating a serious hazard. If the water rose far enough to top the slide and wash it out, severe flooding would occur downriver.

The lake backing up behind the Thistle Slide covers U.S. Highway 6 and has drowned most of the small town of Thistle. One building stands on higher ground, but it too will succumb to the lake in another few days.

By June the slide had stopped moving, and the lake has expanded to reach its maximum size. A drainage tunnel was installed to prevent the water from over topping the landslide. Today, only a faint shoreline, like a ring-around-the-tub, is visible to those who look for it.

City Creek, which flowed through Salt Lake Valley when the Mormon Pioneers arrived, had been long buried underground in conduits under the city. But in June 1983, the creek could not be contained in its underground path and was directed down through the avenues and out onto State Street along the roads.

At North Temple and 2nd Avenue, City Creek was turned to the south by a high wall of sandbags holding up concrete barriers directing the flow onto State Street in Salt Lake City.

City Creek turned to the south past the LDS Church Office Building onto State Street.

City Creek flowing down the east side of State Street and under the Eagle Gate.

Temporary pedestrian bridges were built along State Street to allow people to cross the river now flowing along the pavement.

In Provo, Utah, the normally dry creek that flows out of Rock Canyon flooded neighborhoods in the area around the Provo LDS Temple. Here Rock Canyon Creek flows down the road on the south side of the Provo Temple toward the Missionary Training Center.

As the flow approached the Missionary Training Center, it was diverted onto 9th East, although by the time I took this photo, a break in the main sandbag channel was allowing some of the flow to head directly toward the MTC.

Farther down 9th east at the corner of the BYU Campus, the water was spread out acoss the entire road and was only a few inches deep on this particular day. And check out that red car. You won't see one of those on the road very often these days.

You can read more about the Thistle Landslide at these sites: